BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 6. AMINO ACID METABOLISM AND FUNCTIONS. PROTEIN BIOSYNTHESIS

6.10. Protein biosynthesis

6.10.8. Energetics of translation. Inhibitors of protein synthesis

The Energy balance of Translation indicates the expenditure of a significant amount of Free energy. The formation of aminoacyl-tRNA, as well as proofreading processes during Amino Acid Activation, utilize high-energy ATP bonds, whereas the initiation, elongation, and termination stages involve the Cleavage of GTP into GDP and Pi. According to A. Lehninger, The Biosynthesis of a single peptide bond requires 122 kJ/mol, whereas the Standard Free Energy of peptide bond Hydrolysis is 21 kJ/mol. This excess energy ensures the high fidelity and reliability of biological Introduction/27.html">Translation of the Genetic Code into the Amino Acid Sequence of a protein molecule.

Polyribosomes. Translation of the mRNA chain proceeds from the 5' to the 3' end, allowing a new ribosome to attach to the vacated 5' end and initiate the synthesis of another polypeptide chain. Ribosomes simultaneously synthesizing Proteins on a single mRNA molecule form complexes known as Polysomes (polyribosomes). The ribosomes are positioned along the template at intervals of approximately 100 NUCLEOTIDES. The existence of polyribosome systems significantly enhances the efficiency of mRNA utilization.

Inhibitors of Protein Synthesis. The functioning of template-directed synthesis processes is frequently investigated using specific inhibitors targeting particular stages. There are numerous specific inhibitors, primarily Antibiotics, that selectively block either prokaryotic or eukaryotic RNA or protein synthesis. Many effective antibiotics (drugs) are designed primarily with regard to their impact on the prokaryotic synthesis system.

Tetracyclines inhibit translation at the level of site-specific interaction on the ribosome, specifically by blocking the codon-dependent binding of aminoacyl-tRNA to the A-site of the bacterial 70S ribosome (30S subunit), thereby disrupting Polypeptide chain elongation.

Certain inhibitors suppress primarily the interaction of aminoacyl-tRNA with the ribosome by blocking factor-binding sites on the 50S and 60S subunits. A typical representative of such inhibitors is the antibiotic siomycin.

The antibiotic fusidic acid is a specific inhibitor of translocation. It does not act directly on the ribosome; instead, it binds to the EF-G protein factor, increases the affinity of the latter for the factor-binding site of the 50S subunit, and remains bound following translocation and GTP hydrolysis. Consequently, fusidic acid inhibits the subsequent recruitment and binding of aminoacyl-tRNA at the A-site.

Chloramphenicol blocks the peptidyl transferase reaction within the ribosome.

Streptomycin inhibits initiation by hindering the transition from initiation complex formation to the operation of the active translating ribosome.

Puromycin, which structurally resembles the 3' end of aminoacyl-tRNA, promotes chain termination and premature cessation of translation, because peptidyl-puromycin lacks the anticodon triplet and cannot participate in translocation.

Actinomycin D inhibits synthesis at the level of RNA polymerase activity.

Cycloheximide blocks the translocation reaction on ribosomes.

Viruses can also be classified as inhibitors, since host nucleic acid and protein synthesis are halted in infected Cells. Toxins inhibit synthetic processes at both the transcriptional level (α-amanitin) and the translational level (ricin, diphtheria toxin). The latter is cleaved into two fragments, one of which catalyzes the ADP-ribosylation of the eukaryotic elongation factor eEF-2, leading to its inactivation and the disruption of translocation. The principal symptoms of diphtheria are associated with the action of this toxin. Interferons also act as inhibitors, as they suppress the synthesis of proteins required for viral Replication by both activating ribonucleases and stimulating the synthesis of protein Kinases, which phosphorylate the α-subunit of eIF-2, thereby inactivating it and blocking Translation initiation.

Currently, numerous representatives of various classes of Protein Synthesis Inhibitors have been investigated, which has clarified several fundamental aspects regarding the functioning of specific Components of the complex translation machinery.